Evidently something in the future still depends on him having kids. I suspect the time traveller is a descendant of MC, thus he needs to be born for the whole set of events to take place. In fact, perhaps causal paradoxes can be solved that way. So, while changes in the past can be made, these changes cannot prevent the tome traveller from travelling back. In this case, Raito was always going to become a time traveller and travel back, so he can make changes to the timeline, so long as those changes don't prevent his travelling back. Interesting.